Method of making light emitting device with silicon-containing encapsulant

ABSTRACT

A method of making a light emitting device is disclosed. The method includes the steps of providing a light emitting diode and forming an encapsulant in contact with the light emitting diode; wherein forming the encapsulant includes contacting the light emitting diode with a photopolymerizable composition consisting of a silicon-containing resin and a metal-containing catalyst, wherein the silicon-containing resin consists of silicon-bonded hydrogen and aliphatic unsaturation, and applying actinic radiation having a wavelength of 700 nm or less to initiate hydrosilylation within the silicon-containing resin.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 10/993,460, filed Nov. 18, 2004, now pending.

FIELD OF THE INVENTION

The invention relates to a method of making a light emitting device.More particularly, the invention relates to a method of making a lightemitting device having a light emitting diode (LED) and asilicon-containing encapsulant.

BACKGROUND

Typical encapsulants for LEDs are organic polymeric materials.Encapsulant lifetime is a significant hurdle holding back improvedperformance of high brightness LEDs. Conventional LEDs are encapsulatedin epoxy resins and, when in use, tend to yellow over time reducing theLED brightness and changing the color rendering index of the lightemitted from the light emitting device. This is particularly importantfor white LEDs. The yellowing of the epoxy is believed to result fromdecomposition induced by the high operating temperatures of the LEDand/or absorption of UV-blue light emitted by the LED.

A second problem that can occur when using conventional epoxy resins isstress-induced breakage of the wire bond on repeated thermal cycling.High brightness LEDs can have heat loads on the order of 100 Watts persquare centimeter. Since the coefficients of thermal expansion of epoxyresins typically used as encapsulants are significantly larger thanthose of the semiconductor layers and the moduli of the epoxies can behigh, the embedded wire bond can be stressed to the point of failure onrepeated heating and cooling cycles.

Thus, there is a need for new photochemically stable and thermallystable encapsulants for LEDs that reduce the stress on the wire bondover many temperature cycles. In addition, there is a need forencapsulants with relatively rapid cure mechanisms in order toaccelerate manufacturing times and reduce overall LED cost.

SUMMARY

A method of making a light emitting device is disclosed herein. Themethod includes the steps of providing a LED and forming an encapsulantin contact with the LED; wherein forming the encapsulant includescontacting the LED with a photopolymerizable composition consisting of asilicon-containing resin and a metal-containing catalyst, wherein thesilicon-containing resin consists of silicon-bonded hydrogen andaliphatic unsaturation, and applying actinic radiation having awavelength of 700 nm or less to initiate hydrosilylation within thesilicon-containing resin. The method disclosed herein may furthercomprise the step of heating at less than 150° C.

Light emitting devices which may be prepared according to the abovemethods are also disclosed herein, in addition to a light emittingdevice comprising a light emitting diode and a photopolymerizablecomposition comprising a silicon-containing resin and a metal-containingcatalyst, wherein the silicon-containing resin comprises silicon-bondedhydrogen and aliphatic unsaturation.

The encapsulant described herein can have one or more of the followingdesirable features: high refractive index, photochemical stability,thermal stability, formable by relatively rapid cure mechanisms, andformable at relatively low temperatures.

These and other aspects of the invention will be apparent from thedetailed description below. In no event, however, should the abovesummary be construed as a limitation on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF THE DRAWING

The invention may be more completely understood in consideration of thefollowing detailed description and examples in connection with theFIGURE described below. The FIGURE is an illustrative example and, in noevent, should be construed as a limitation on the claimed subjectmatter, which subject matter is defined solely by the claims set forthherein.

The FIGURE is a schematic diagram of a light emitting device capable ofbeing prepared according to the disclosed method.

DETAILED DESCRIPTION

A method of making a light emitting device is disclosed. Referring tothe FIGURE, LED 1 is mounted on a metallized contact 2 a disposed on asubstrate 6 in a reflecting cup 3. LED 1 has one electrical contact onits lowermost surface and another on its uppermost surface, the latterof which is connected to a separate electrical contact 2 b by a wirebond 4. A power source can be coupled to the electrical contacts toenergize the LED. Encapsulant 5 encapsulates the LED.

A method for preparing a light emitting device with an LED sealed withina silicon-containing encapsulant is disclosed. Such silicon-containingencapsulants are advantageous because of their thermal and photochemicalstability. Silicon-containing encapsulants are known in the art. Thesecompositions typically comprise organosiloxanes that are cured either byacid-catalyzed condensation reactions between silanol groups bonded tothe organosiloxane components or by metal-catalyzed hydrosilylationreactions between groups incorporating aliphatic unsaturation andsilicon-bonded hydrogen, which are bonded to the organosiloxanecomponents. In the first instance, the curing reaction is relativelyslow, sometimes requiring many hours to proceed to completion. In thesecond instance, desirable levels of cure normally require temperaturessignificantly in excess of room temperature. For example, US PatentApplication Publication US 2004/0116640 A1 states that such compositionsare “ . . . preferably cured by heating at about 120 to 180° C. forabout 30 to 180 minutes.”

The method disclosed herein also utilizes organosiloxane compositionsthat are cured by metal-catalyzed hydrosilylation reactions betweengroups incorporating aliphatic unsaturation and silicon-bonded hydrogen,which are bonded to the organosiloxane components. The metal-containingcatalysts used herein can be activated by actinic radiation. Theadvantages of initiating hydrosilylation using catalysts activated byactinic radiation include (1) the ability to cure the encapsulatingcomposition without subjecting the LED, the substrate to which it isattached, or any other materials present in the package or system, topotentially harmful temperatures, (2) the ability to formulate one-partencapsulating compositions that display long working times (also knownas bath life or shelf life), (3) the ability to cure the encapsulatingformulation on demand at the discretion of the user, and (4) the abilityto simplify the formulation process by avoiding the need for two-partformulations as is typically required for thermally curablehydrosilylation compositions.

The disclosed method involves the use of actinic radiation having awavelength of less than or equal to 700 nanometers (nm). Thus, thedisclosed methods are particularly advantageous to the extent they avoidharmful temperatures. Preferably, the disclosed methods involve theapplication of actinic radiation at a temperature of less than 120° C.,more preferably, at a temperature of less than 60° C., and still morepreferably at a temperature of 25° C. or less.

Actinic radiation used in the disclosed methods includes light of a widerange of wavelengths less than or equal to 700 nm, including visible andUV light, but preferably, the actinic radiation has a wavelength of of600 nm or less, and more preferably from 200 to 600 nm., and even morepreferably, from 250 to 500 nm. Preferably, the actinic radiation has awavelength of at least 200 nm, and more preferably at least 250 nm.

A sufficient amount of actinic radiation is applied to thesilicon-containing resin for a time to form an at least partially curedencapsulant. A partially cured encapsulant means that at least 5 molepercent of the aliphatic unsaturation is consumed in a hydrosilylationreaction. Preferably, a sufficient amount of the actinic radiation isapplied to the silicon-containing resin for a time to form asubstantially cured encapsulant. A substantially cured encapsulant meansthat greater than 60 mole percent of the aliphatic unsaturation presentin the reactant species prior to reaction has been consumed as a resultof the light activated addition reaction of the silicon-bonded hydrogenwith the aliphatic unsaturated species. Preferably, such curing occursin less than 30 minutes, more preferably in less than 10 minutes, andeven more preferably in less than 5 minutes or less than 1 minute. Incertain embodiments, such curing can occur in less than 10 seconds.

Examples of sources of actinic radiation include tungsten halogen lamps,xenon arc lamps, mercury arc lamps, incandescent lamps, germicidallamps, and fluorescent lamps. In certain embodiments, the source ofactinic radiation is the LED.

In some cases, the method disclosed herein may further comprise the stepof heating after actinic radiation is applied to form the encapsulant.Actinic radiation may be applied to gel the silicon-containing resin andcontrol settling of any additional components such as particles,phosphors, etc. which may be present in the encapsulant. Controlledsettling of the particles or phosphors may be used to achieve specificuseful spatial distributions of the particles or phosphors within theencapsulant. For example, the method may allow controlled settling ofparticles enabling formation of a gradient refractive index distributionthat may enhance LED efficiency or emission pattern. It may also beadvantageous to allow partial settling of phosphor such that a portionof the encapsulant is clear and other portions contain phosphor. In thiscase, the clear portion of encapsulant can be shaped to act as a lensfor the emitted light from the phosphor.

Other than to control settling, the step of heating after actinicradiation is applied may be used to accelerate formation of theencapsulant, or to decrease the amount of time the encapsulant isexposed to actinic radiation during the previous step. Any heating meansmay be used such as an infrared lamp, a forced air oven, or a heatingplate. If applied, heating may be at less than 150° C., or morepreferably at less than 100° C., and still more preferably at less than60° C.

Also disclosed herein is a light emitting device comprising a lightemitting diode and a photopolymerizable composition comprising asilicon-containing resin and a metal-containing catalyst, wherein thesilicon-containing resin comprises silicon-bonded hydrogen and aliphaticunsaturation. In some embodiments, the metal-containing catalyst maycomprise platinum. In other embodiments, the photopolymerizablecomposition may be at a temperature of from about 30° C. to about 120°C. In other embodiments, the metal-containing catalyst may compriseplatinum, and the photopolymerizable composition may be at a temperatureof from about 30° C. to about 120° C.

In some cases, the method disclosed herein may further comprise the stepof heating at a temperature of from about 30° C. to about 120° C. beforeactinic radiation is applied. Heating may be carried out in order tolower the viscosity of the photopolymerizable composition, for example,to facilitate the release of any entrapped gas. Heat may optionally beapplied during or after application of the actinic radiation toaccelerate formation of the encapsulant.

The silicon-containing resin can include monomers, oligomers, polymers,or mixtures thereof. It includes silicon-bonded hydrogen and aliphaticunsaturation, which allows for hydrosilylation (i.e., the addition of asilicon-bonded hydrogen across a carbon-carbon double bond or triplebond). The silicon-bonded hydrogen and the aliphatic unsaturation may ormay not be present in the same molecule. Furthermore, the aliphaticunsaturation may or may not be directly bonded to silicon.

Preferred silicon-containing resins are those that provide anencapsulant, which can be in the form of a liquid, gel, elastomer, or anon-elastic solid, and are thermally and photochemically stable. For UVlight, silicon-containing resins having refractive indices of at least1.34 are preferred. For some embodiments, silicon-containing resinshaving refractive indices of at least 1.50 are preferred.

Preferred silicon-containing resins are selected such that they providean encapsulant that is photostable and thermally stable. Herein,photostable refers to a material that does not chemically degrade uponprolonged exposure to actinic radiation, particularly with respect tothe formation of colored or light absorbing degradation products.Herein, thermally stable refers to a material that does not chemicallydegrade upon prolonged exposure to heat, particularly with respect tothe formation of colored or light absorbing degradation products. Inaddition, preferred silicon-containing resins are those that possessrelatively rapid cure mechanisms (e.g., seconds to less than 30 minutes)in order to accelerate manufacturing times and reduce overall LED cost.

Examples of suitable silicon-containing resins are disclosed, forexample, in U.S. Pat. No. 6,376,569 (Oxman et al.), U.S. Pat. No.4,916,169 (Boardman et al.), U.S. Pat. No. 6,046,250 (Boardman et al.),U.S. Pat. No. 5,145,886 (Oxman et al.), U.S. Pat. No. 6,150,546 (Butts),and in U.S. Pat. Appl. Nos. 2004/0116640 (Miyoshi). A preferredsilicon-containing resin comprises an organosiloxane (i.e., silicones),which includes organopolysiloxanes. Such resins typically include atleast two components, one having silicon-bonded hydrogen and one havingaliphatic unsaturation. However, both silicon-bonded hydrogen andolefinic unsaturation may exist within the same molecule.

In one embodiment, the silicon-containing resin can include a siliconecomponent having at least two sites of aliphatic unsaturation (e.g.,alkenyl or alkynyl groups) bonded to silicon atoms in a molecule and anorganohydrogensilane and/or organohydrogenpolysiloxane component havingat least two hydrogen atoms bonded to silicon atoms in a molecule.Preferably, a silicon-containing resin includes both components, withthe silicone containing aliphatic unsaturation as the base polymer(i.e., the major organosiloxane component in the composition.) Preferredsilicon-containing resins are organopolysiloxanes. Such resins typicallycomprise at least two components, at least one of which containsaliphatic unsaturation and at least one of which contains silicon-bondedhydrogen. Such organopolysiloxanes are known in the art and aredisclosed in such patents as U.S. Pat. No. 3,159,662 (Ashby), U.S. Pat.No. 3,220,972 (Lamoreauz), U.S. Pat. No. 3,410,886 (Joy), U.S. Pat. No.4,609,574 (Keryk), U.S. Pat. No. 5,145,886 (Oxman, et. al), and U.S.Pat. No. 4,916,169 (Boardman et. al). Curable one componentorganopolysiloxane resins are possible if the single resin componentcontains both aliphatic unsaturation and silicon-bonded hydrogen.

Organopolysiloxanes that contain aliphatic unsaturation are preferablylinear, cyclic, or branched organopolysiloxanes comprising units of theformula R¹ _(a)R² _(b)SiO_((4-a-b)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substitutedhydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; R² is a monovalent hydrocarbon group havingaliphatic unsaturation and from 2 to 10 carbon atoms; a is 0, 1, 2, or3; b is 0, 1, 2, or 3; and the sum a+b is 0, 1, 2, or 3; with theproviso that there is on average at least 1 R² present per molecule.

Organopolysiloxanes that contain aliphatic unsaturation preferably havean average viscosity of at least 5 mPa·s at 25° C.

Examples of suitable R¹ groups are alkyl groups such as methyl, ethyl,n-propyl, iso-propyl, n-butyl, iso-butyl, tert-butyl, n-pentyl,iso-pentyl, neo-pentyl, tert-pentyl, cyclopentyl, n-hexyl, cyclohexyl,n-octyl, 2,2,4-trimethylpentyl, n-decyl, n-dodecyl, and n-octadecyl;aromatic groups such as phenyl or naphthyl; alkaryl groups such as4-tolyl; aralkyl groups such as benzyl, 1-phenylethyl, and2-phenylethyl; and substituted alkyl groups such as3,3,3-trifluoro-n-propyl, 1,1,2,2-tetrahydroperfluoro-n-hexyl, and3-chloro-n-propyl.

Examples of suitable R² groups are alkenyl groups such as vinyl,5-hexenyl, 1-propenyl, allyl, 3-butenyl, 4-pentenyl, 7-octenyl, and9-decenyl; and alkynyl groups such as ethynyl, propargyl and 1-propynyl.In the present invention, groups having aliphatic carbon-carbon multiplebonds include groups having cycloaliphatic carbon-carbon multiple bonds.

Organopolysiloxanes that contain silicon-bonded hydrogen are preferablylinear, cyclic or branched organopolysiloxanes comprising units of theformula R¹ _(a)H_(c)SiO_((4-a-c)/2) wherein: R¹ is as defined above; ais 0, 1, 2, or 3; c is 0, 1, or 2; and the sum of a+c is 0, 1, 2, or 3;with the proviso that there is on average at least 1 silicon-bondedhydrogen atom present per molecule.

Organopolysiloxanes that contain silicon-bonded hydrogen preferably havean average viscosity of at least 5 mPa·s at 25° C.

Organopolysiloxanes that contain both aliphatic unsaturation andsilicon-bonded hydrogen preferably comprise units of both formulae R¹_(a)R² _(b)SiO_((4-a-b)/2) and R¹ _(a)H_(c)SiO_((4-a-c)/2). In theseformulae, R¹, R², a, b, and c are as defined above, with the provisothat there is an average of at least 1 group containing aliphaticunsaturation and 1 silicon-bonded hydrogen atom per molecule.

The molar ratio of silicon-bonded hydrogen atoms to aliphaticunsaturation in the silicon-containing resin (particularly theorganopolysiloxane resin) may range from 0.5 to 10.0 mol/mol, preferablyfrom 0.8 to 4.0 mol/mol, and more preferably from 1.0 to 3.0 mol/mol.

For some embodiments, organopolysiloxane resins described above whereina significant fraction of the R¹ groups are phenyl or other aryl,aralkyl, or alkaryl are preferred, because the incorporation of thesegroups provides materials having higher refractive indices thanmaterials wherein all of the R¹ radicals are, for example, methyl.

The disclosed compositions also include a metal-containing catalystwhich enables the cure of the encapulating material viaradiation-activated hydrosilylation. These catalysts are known in theart and typically include complexes of precious metals such as platinum,rhodium, iridium, cobalt, nickel, and palladium. The preciousmetal-containing catalyst preferably contains platinum. Disclosedcompositions can also include a cocatalyst, i.e., the use of two or moremetal-containing catalysts.

A variety of such catalysts is disclosed, for example, in U.S. Pat. No.6,376,569 (Oxman et al.), U.S. Pat. No. 4,916,169 (Boardman et al.),U.S. Pat. No. 6,046,250 (Boardman et al.), U.S. Pat. No. 5,145,886(Oxman et al.), U.S. Pat. No. 6,150,546 (Butts), U.S. Pat. No. 4,530,879(Drahnak), U.S. Pat. No. 4,510,094 (Drahnak) U.S. Pat. No. 5,496,961(Dauth), U.S. Pat. No. 5,523,436 (Dauth), U.S. Pat. No. 4,670,531(Eckberg), as well as International Publication No. WO 95/025735(Mignani).

Certain preferred platinum-containing catalysts are selected from thegroup consisting of Pt(II) β-diketonate complexes (such as thosedisclosed in U.S. Pat. No. 5,145,886 (Oxman et al.),(η⁵-cyclopentadienyl)tri(σ-aliphatic)platinum complexes (such as thosedisclosed in U.S. Pat. No. 4,916,169 (Boardman et al.) and U.S. Pat. No.4,510,094 (Drahnak)), and C₇₋₂₀-aromatic substituted(η⁵-cyclopentadienyl)tri(σ-aliphatic)platinum complexes (such as thosedisclosed in U.S. Pat. No. 6,150,546 (Butts).

Such catalysts are used in an amount effective to accelerate thehydrosilylation reaction. Such catalysts are preferably included in theencapsulating material in an amount of at least 1 part, and morepreferably at least 5 parts, per one million parts of the encapsulatingmaterial composition. Such catalysts are preferably included in theencapsulating material in an amount of no greater than 1000 parts ofmetal, and more preferably no greater than 200 parts of metal, per onemillion parts of the encapsulating material composition.

In addition to the silicon-containing resins and catalysts, theencapsulating material can also include nonabsorbing metal oxideparticles, semiconductor particles, phosphors, sensitizers,photoinitiators, antioxidants, catalyst inhibitors, and pigments. Ifused, such additives are used in amounts to produced the desired effect.

Particles that are included within the encapsulating material can besurface treated to improve dispersibility of the particles in the resin.Examples of such surface treatment chemistries include silanes,siloxanes, carboxylic acids, phosphonic acids, zirconates, titanates,and the like. Techniques for applying such surface treatment chemistriesare known.

Nonabsorbing metal oxide and semiconductor particles can optionally beincluded in the encapsulating material to increase the refractive indexof the encapsulant. Suitable nonabsorbing particles are those that aresubstantially transparent over the emission bandwidth of the LED.Examples of nonabsorbing metal oxide and semiconductor particlesinclude, but are not limited to, Al₂O₃, ZrO₂, TiO₂, V₂O₅, ZnO, SnO₂,ZnS, SiO₂, and mixtures thereof, as well as other sufficientlytransparent non-oxide ceramic materials such as semiconductor materialsincluding such materials as ZnS, CdS, and GaN. Silica (SiO₂), having arelatively low refractive index, may also be useful as a particlematerial in some applications, but, more significantly, it can also beuseful as a thin surface treatment for particles made of higherrefractive index materials, to allow for more facile surface treatmentwith organosilanes. In this regard, the particles can include speciesthat have a core of one material on which is deposited a material ofanother type. If used, such nonabsorbing metal oxide and semiconductorparticles are preferably included in the encapsulating material in anamount of no greater than 85 wt-%, based on the total weight of theencapsulating material. Preferably, the nonabsorbing metal oxide andsemiconductor particles are included in the encapsulating material in anamount of at least 10 wt-%, and more preferably in an amount of at least45 wt-%, based on the total weight of the encapsulating material.Generally the particles can range in size from 1 nanometer to 1 micron,preferably from 10 nanometers to 300 nanometers, more preferably, from10 nanometers to 100 nanometers. This particle size is an averageparticle size, wherein the particle size is the longest dimension of theparticles, which is a diameter for spherical particles. It will beappreciated by those skilled in the art that the volume percent of metaloxide and/or semiconductor particles cannot exceed 74 percent by volumegiven a monomodal distribution of spherical particles.

Phosphors can optionally be included in the encapsulating material toadjust the color emitted from the LED. As described herein, a phosphorconsists of a fluorescent material. The fluorescent material could beinorganic particles, organic particles, or organic molecules or acombination thereof. Suitable inorganic particles include doped garnets(such as YAG:Ce and (Y,Gd)AG:Ce), aluminates (such as Sr₂Al₁₄O₂₅:Eu, andBAM:Eu), silicates (such as SrBaSiO:Eu), sulfides (such as ZnS:Ag,CaS:Eu, and SrGa₂S₄:Eu), oxy-sulfides, oxy-nitrides, phosphates,borates, and tungstates (such as CaWO₄). These materials may be in theform of conventional phosphor powders or nanoparticle phosphor powders.Another class of suitable inorganic particles is the so-called quantumdot phosphors made of semiconductor nanoparticles including Si, Ge, CdS,CdSe, CdTe, ZnS, ZnSe, ZnTe, PbS, PbSe, PbTe, InN, InP, InAs, AlN, AlP,AlAs, GaN, GaP, GaAs and combinations thereof. Generally, the surface ofeach quantum dot will be at least partially coated with an organicmolecule to prevent agglomeration and increase compatibility with thebinder. In some cases the semiconductor quantum dot may be made up ofseveral layers of different materials in a core-shell construction.Suitable organic molecules include fluorescent dyes such as those listedin U.S. Pat. No. 6,600,175 (Baretz et al.). Preferred fluorescentmaterials are those that exhibit good durability and stable opticalproperties. The phosphor layer may consist of a blend of different typesof phosphors in a single layer or a series of layers, each containingone or more types of phosphors. The inorganic phosphor particles in thephosphor layer may vary in size (e.g., diameter) and they may besegregated such that the average particle size is not uniform across thecross-section of the siloxane layer in which they are incorporated. Ifused, the phosphor particles are preferably included in theencapsulating material in an amount of no greater than 85 wt-%, and inan amount of at least 1 wt-%, based on the total weight of theencapsulating material. The amount of phosphor used will be adjustedaccording to the thickness of the siloxane layer containing the phosphorand the desired color of the emitted light.

Sensitizers can optionally be included in the encapsulating material toboth increase the overall rate of the curing process (or hydrosilylationreaction) at a given wavelength of initiating radiation and/or shift theoptimum effective wavelength of the initiating radiation to longervalues. Useful sensitizers include, for example, polycyclic aromaticcompounds and aromatic compounds containing a ketone chromaphore (suchas those disclosed in U.S. Pat. No. 4,916,169 (Boardman et al.) and U.S.Pat. No. 6,376,569 (Oxman et al.)). Examples of useful sensitizersinclude, but are not limited to, 2-chlorothioxanthone,9,10-dimethylanthracene, 9,10-dichloroanthracene, and2-ethyl-9,10-dimethylanthracene. If used, such sensitizers arepreferably included in the encapsulating material in an amount of nogreater than 50,000 parts by weight, and more preferably no greater than5000 parts by weight, per one million parts of the composition. If used,such sensitizers are preferably included in the encapsulating materialin an amount of at least 50 parts by weight, and more preferably atleast 100 parts by weight, per one million parts of the composition.

Photoinitiators can optionally be included in the encapsulating materialto increase the overall rate of the curing process (or hydrosilylationreaction). Useful photoinitiators include, for example, monoketals ofα-diketones or α-ketoaldehydes and acyloins and their correspondingethers (such as those disclosed in U.S. Pat. No. 6,376,569 (Oxman etal.)). If used, such photoinitiators are preferably included in theencapsulating material in an amount of no greater than 50,000 parts byweight, and more preferably no greater than 5000 parts by weight, perone million parts of the composition. If used, such photoinitiators arepreferably included in the encapsulating material in an amount of atleast 50 parts by weight, and more preferably at least 100 parts byweight, per one million parts of the composition.

Catalyst inhibitors can optionally be included in the encapsulatingmaterial to further extend the usable shelf life of the composition.Catalyst inhibitors are known in the art and include such materials asacetylenic alcohols (for example, see U.S. Pat. No. 3,989,666 (Niemi)and U.S. Pat. No. 3,445,420 (Kookootsedes et al.)), unsaturatedcarboxylic esters (for example, see U.S. Pat. No. 4,504,645 (Melancon),U.S. Pat. No. 4,256,870 (Eckberg), U.S. Pat. No. 4,347,346 (Eckberg),and U.S. Pat. No. 4,774,111 (Lo)) and certain olefinic siloxanes (forexample, see U.S. Pat. No. 3,933,880 (Bergstrom), U.S. Pat. No.3,989,666 (Niemi), and U.S. Pat. No. 3,989,667 (Lee et al.). If used,such catalyst inhibitors are preferably included in the encapsulatingmaterial in an amount not to exceed the amount of the metal-containingcatalyst on a mole basis.

LEDs

The silicon-containing materials described herein are useful asencapsulants for light emitting devices that include an LED. LED in thisregard refers to a diode that emits light, whether visible, ultraviolet,or infrared. It includes incoherent epoxy-encased semiconductor devicesmarketed as “LEDs”, whether of the conventional or super-radiantvariety. Vertical cavity surface emitting laser diodes are another formof LED. An “LED die” is an LED in its most basic form, i.e., in the formof an individual component or chip made by semiconductor waferprocessing procedures. The component or chip can include electricalcontacts suitable for application of power to energize the device. Theindividual layers and other functional elements of the component or chipare typically formed on the wafer scale, the finished wafer finallybeing diced into individual piece parts to yield a multiplicity of LEDdies.

The silicon-containing materials described herein are useful with a widevariety of LEDs, including monochrome and phosphor-LEDs (in which blueor UV light is converted to another color via a fluorescent phosphor).They are also useful for encapsulating LEDs packaged in a variety ofconfigurations, including but not limited to LEDs surface mounted inceramic or polymeric packages, which may or may not have a reflectingcup, LEDs mounted on circuit boards, and LEDs mounted on plasticelectronic substrates.

LED emission light can be any light that an LED source can emit and canrange from the UV to the infrared portions of the electromagneticspectrum depending on the composition and structure of the semiconductorlayers. Where the source of the actinic radiation is the LED itself, LEDemission is preferably in the range from 350-500 nm. Thesilicon-containing materials described herein are particularly useful insurface mount and side mount LED packages where the encapsulant is curedin a reflector cup. They are also particularly useful with LED designscontaining a top wire bond (as opposed to flip-chip configurations).Additionally, the silicon containing materials can be useful for surfacemount LEDs where there is no reflector cup and can be useful forencapsulating arrays of surface mounted LEDs attached to a variety ofsubstrates.

The silicon-containing materials described herein are resistant tothermal and photodegradation (resistant to yellowing) and thus areparticularly useful for white light sources (i.e., white light emittingdevices). White light sources that utilize LEDs in their constructioncan have two basic configurations. In one, referred to herein as directemissive LEDs, white light is generated by direct emission of differentcolored LEDs. Examples include a combination of a red LED, a green LED,and a blue LED, and a combination of a blue LED and a yellow LED. In theother basic configuration, referred to herein as LED-excitedphosphor-based light sources (PLEDs), a single LED generates light in anarrow range of wavelengths, which impinges upon and excites a phosphormaterial to produce visible light. The phosphor can comprise a mixtureor combination of distinct phosphor materials, and the light emitted bythe phosphor can include a plurality of narrow emission linesdistributed over the visible wavelength range such that the emittedlight appears substantially white to the unaided human eye.

An example of a PLED is a blue LED illuminating a phosphor that convertsblue to both red and green wavelengths. A portion of the blue excitationlight is not absorbed by the phosphor, and the residual blue excitationlight is combined with the red and green light emitted by the phosphor.Another example of a PLED is an ultraviolet (UV) LED illuminating aphosphor that absorbs and converts UV light to red, green, and bluelight. Organopolysiloxanes where the R¹ groups are small and haveminimal UV absorption, for example methyl, are preferred for UV lightemitting diodes. It will be apparent to one skilled in the art thatcompetitive absorption of the actinic radiation by the phosphor willdecrease absorption by the photoinitiators slowing or even preventingcure if the system is not carefully constructed.

EXAMPLES

LED Package 1: Mounting Blue LED Die in a Ceramic Package

Into a Kyocera package (Kyocera America, Inc., Part No. KD-LA2707-A) wasbonded a Cree XB die (Cree Inc., Part No. C460XB290-0103-A) using awater based halide flux (Superior No. 30, Superior Flux & Mfg. Co.). TheLED device was completed by wire bonding (Kulicke and Soffa Industries,Inc. 4524 Digital Series Manual Wire Bonder) the Cree XB die using 1 milgold wire. Prior to encapsulation, each device was tested using a OL 770Spectroradiometer (Optronics Laboratories, Inc.) with a constant currentof 20 mA. The peak emission wavelength of the LED was 455-457 nm.

Preparation of Organopolysiloxane

The organopolysiloxane H₂C═CH—Si(CH₃)₂O—[Si(CH₃)₂O]₁₀₀—Si(CH₃)₂—CH═CH₂was prepared as follows. In a half-gallon polyethylene bottle werecombined 1000.0 g (3.371 mol) of octamethylcyclotetrasiloxane (Gelest,Inc.), 25.1 g (0.135 mol) of 1,3-divinyl-1,1,3,3,-tetramethyldisiloxane(Gelest, Inc.), 1.0 g of concentrated sulfuric acid, and 5.0 g ofactivated carbon. The mixture was agitated at room temperature for 24hours and filtered. Volatiles were separated from the filtrate at 200°C. using a thin film evaporator to give 870.0 g of a clear, colorlessfluid. The ¹H and ²⁹Si NMR spectra of the product were consistent withthe structure of the desired organopolysiloxane.

Example 1 Visible Light Cure

A mixture of siloxanes consisting of 10.00 g (olefin meq wt=3.801 g) ofthe organopolysiloxane prepared as described in the previous paragraphand 0.44 g (Si—H meq wt=0.111 g) of(CH₃)₃SiO—[Si(CH₃)₂O]₁₅—[SiH(CH₃)O]₂₅—Si(CH₃)₃ (Dow Corning Corp.,SYL-OFF® 7678) was prepared in a 35 mL amber bottle. A catalyst stocksolution was prepared by dissolving 22.1 mg of Pt(acac)₂ (wherein acacis acetoacetonate, purchased from Aldrich Chemical Co.) in 1.00 mL ofCH₂Cl₂. A 100 μL aliquot of this catalyst stock solution was added tothe mixture of siloxanes. The final formulation was equivalent to aratio of aliphatic unsaturation to silicon-bonded hydrogen of 1.5 andcontained approximately 100 ppm of platinum.

Into LED Package 1 was placed approximately 2 mg of the finalformulation described above. The LED was illuminated for 2.5 minutes at20 mA. The encapsulated device was allowed to sit for an additional 5minutes. The encapsulant was elastomeric and cured as determined byprobing with the tip of a tweezer. The efficiency of the resultingencapsulated LED device was measured using an OL 770 spectroradiometerand increased from 9.3% before encapsulation to 11.8% afterencapsulation.

Example 2 UV Light Cure

An encapsulated LED device was prepared and evaluated in the same manneras described in Example 1 except that 21.1 mg of CpPt(CH₃)₃, prepared asdescribed in Boardman et al., Magn. Reson. Chem., 30, 481 (1992), wasused instead of 22.1 mg of Pt(acac)₂, and illumination was carried outusing a UV lamp at 365 nm. Efficiency increased from 8.9% beforeencapsulation to 11.6% after encapsulation.

FURTHER EMBODIMENTS

Examples 3-6 illustrate further embodiments of the invention that can bemade.

Example 3 Visible Light Cure

A mixture of siloxanes consisting of 10.00 g (olefin meq wt=1.46 g) ofthe vinyl siloxane base polymerH₂C═CH—Si(CH₃)₂O—[Si(CH₃)(C₆H₅)O]_(n)—Si(CH₃)₂—CH═CH₂ (Gelest, Inc.,PMV-9925) and 1.64 g (Si—H meq wt=0.16 g) of the siloxane crosslinkingagent H(CH₃)₂SiO—[SiH(CH₃)O]_(m)—[Si(CH₃)(C₆H₅)O]_(n)—Si(CH₃)₂H (Gelest,Inc., HPM-502) is prepared in a 35 mL amber bottle. A 100 μL aliquot ofa Pt(acac)₂ solution in CH₂Cl₂ prepared as described in Example 1 isadded to the mixture of siloxanes. The final formulation is equivalentto a ratio of aliphatic unsaturation to silicon-bonded hydrogen of 1.5and contains approximately 100 ppm of platinum.

Into LED Package 1 is placed approximately 2 mg of the above finalformulation. The LED device is illuminated for 2.5 minutes at 20 mA andthen allowed to sit for an additional 5 minutes. The encapsulant iselastomeric and cured as determined by probing with the tip of atweezer.

Example 4 UV Light Cure

An encapsulated LED device is prepared and evaluated in the same manneras described in Example 3 except that 21.1 mg of CpPt(CH₃)₃ is usedinstead of 22.1 mg of Pt(acac)₂, and illumination is carried out using aUV lamp at 365 nm. The encapsulant is elastomeric and cured asdetermined by probing with the tip of a tweezer.

Example 5 Visible Light Cure

A mixture of siloxanes consisting of 10.00 g (olefin meq wt=1.24 g) ofthe vinyl silsesquioxane base polymer[H₂C═CH—SiO_(3/2)]_(m)-[Si(C₆H₅)O_(3/2)]_(n) (where m and n represent 10and 90 mole percent, respectively, of the monomer units in thesilsesquioxane; Gelest, Inc., SST-3PV1) and 1.92 g (Si—H meq wt=0.16 g)of the siloxane crosslinking agentH(CH₃)₂SiO—[Si(C₆H₅)[OSi(CH₃)₂H]O]_(n)—Si(CH₃)₂H (Gelest, Inc., HDP-111)is prepared in a 35 mL amber bottle. A 100 μL aliquot of a Pt(acac)₂solution in CH₂Cl₂ prepared as described in Example 1 is added to themixture of siloxanes. The final formulation is equivalent to a ratio ofaliphatic unsaturation to silicon-bonded hydrogen of 1.5 and containsapproximately 100 ppm of platinum.

Into LED Package 1 is placed approximately 2 mg of the above finalformulation. The LED is illuminated for 2.5 minutes at 20 mA and thenallowed to sit for an additional 5 minutes. The encapsulant iselastomeric and cured as determined by probing with the tip of atweezer.

Example 6 UV Light Cure

An encapsulated LED device is prepared and evaluated in the same manneras described in Example 5 except that 21.1 mg of CpPt(CH₃)₃ is usedinstead of 22.1 mg of Pt(acac)₂, and illumination is carried out using aUV lamp at 365 nm. The encapsulant is elastomeric and cured asdetermined by probing with the tip of a tweezer.

Additional Examples

LED Package 2: Mounting Blue LED Die in a Ceramic Package

Into a Kyocera package (Kyocera America, Inc., Part No. KD-LA2707-A) wasbonded a Cree XT die (Cree Inc., Part No. C460XT290-0119-A) using awater based halide flux (Superior No. 30, Superior Flux & Mfg. Co.). TheLED device was completed by wire bonding (Kulicke and Soffa Industries,Inc. 4524 Digital Series Manual Wire Bonder) the Cree XT die using 1 milgold wire. Prior to encapsulation, each device was tested using an OL770 Spectroradiometer (Optronics Laboratories, Inc.) with a constantcurrent of 20 mA. The peak emission wavelength of the LED was 458-460nm.

Example 7

A mixture of siloxanes consisting of 10.00 g of the vinyl siloxane basepolymer H₂C═CH—Si(CH₃)₂O—[Si(CH₃)₂O]₈₀—[Si(C₆H₅)₂O]₂₆—Si(CH₃)₂—CH═CH₂(purchased from Gelest as PDV-2331) and 1.04 g ofH(CH₃)₂SiO—[Si(CH₃)HO]₁₅—[Si(CH₃)(C₆H₅)O]₁₅—Si(CH₃)₂H (purchased fromGelest as HPM-502) was prepared in a 35 mL amber bottle. A 100 μLaliquot of a solution of 33 mg of CH₃ CpPt(CH₃)₃ in 1 mL of toluene wasadded to the mixture of siloxanes, the mixture was degassed undervacuum, and the final composition was labeled Encapsulant A.

Into LED Package 2 was placed a small drop of Encapsulant A using thetip of a syringe needle such that the LED and wire bond were covered andthe device was filled to level to the top of the reflector cup. Thesiloxane encapsulant was irradiated for 3 minutes under a UVP Blak-RayLamp Model XX-15 fitted with two 16-inch Philips F 15T8/BL 15W bulbsemitting at 365 nm from a distance of 20 mm from the encapsulated LED.The encapsulant was judged fully cured, tack free and elastomeric byprobing with the tip of a tweezer.

Example 8

A blue LED device was filled with Encapsulant A as described in Example7. The siloxane encapsulant was irradiated as described in Example 1 butonly for 15 seconds. The filled LED device containing the irradiatedencapsulant was then placed on a hotplate set at 100° C. After 1 minutethe encapsulant was judged fully cured, tack free and elastomeric byprobing with the tip of a tweezer. Prior to heating at 100° C. theencapsulant was an incompletely cured tacky gel.

Example 9

A blue LED device was filled with Encapsulant A as described in Example7. The siloxane encapsulant was irradiated as described in Example 1 butonly for 15 seconds. The filled LED device containing the irradiatedencapsulant was then allowed to stand at room temperature. After 10minutes the encapsulant was judged fully cured, tack free andelastomeric by probing with the tip of a tweezer. Prior to standing atroom temperature, the encapsulant was an incompletely cured tacky gel.

Control Example

A blue LED device was filled with Encapsulant A as described in Example7. The siloxane-filled LED device was placed on a hotplate set at 100°C. After 20 minutes the encapsulant was still liquid and showed noindication of cure as judged by probing with the tip of a tweezer.

The complete disclosures of the patents, patent documents, andpublications cited herein are incorporated by reference in theirentirety as if each were individually incorporated. Variousmodifications and alterations to the invention will become apparent tothose skilled in the art without departing from the scope and spirit ofthe invention. It should be understood that the invention is notintended to be unduly limited by the illustrative embodiments andexamples set forth herein, and that such examples and embodiments arepresented by way of example only with the scope of the inventionintended to be limited only by the claims set forth herein as follows.

1. A method of making a light emitting device, the method comprising thesteps of: providing a light emitting diode; and forming an encapsulantin contact with the light emitting diode, wherein forming theencapsulant comprises: contacting the light emitting diode with aphotopolymerizable composition comprising a silicon-containing resin anda metal-containing catalyst, wherein the silicon-containing resincomprises silicon-bonded hydrogen and aliphatic unsaturation; andapplying actinic radiation having a wavelength of 700 nm or less toinitiate hydrosilylation within the silicon-containing resin.
 2. Themethod of claim 1 wherein the silicon-bonded hydrogen and the aliphaticunsaturation are present in the same molecule.
 3. The method of claim 1wherein the silicon-bonded hydrogen and the aliphatic unsaturation arepresent in different molecules.
 4. The method of claim 1 wherein atleast 5 mole percent of the aliphatic unsaturation is consumed in ahydrosilylation reaction.
 5. The method of claim 1 wherein at least 60mole percent of the aliphatic unsaturation is consumed in ahydrosilylation reaction.
 6. The method of claim 5 wherein thehydrosilylation reaction occurs in less than 30 minutes.
 7. The methodof claim 6 wherein the hydrosilylation reaction occurs in less than 10minutes.
 8. The method of claim 7 wherein the hydrosilylation reactionoccurs in less than 5 minutes.
 9. The method of claim 8 wherein thehydrosilylation reaction occurs in less than 1 minute.
 10. The method ofclaim 9 wherein the hydrosilylation reaction occurs in less than 10seconds.
 11. The method of claim 1 wherein applying actinic radiationcomprises activating the light emitting diode.
 12. The method of claim 1wherein applying actinic radiation comprises applying actinic radiationat a temperature of less than 120° C.
 13. The method of claim 12 whereinapplying actinic radiation comprises applying actinic radiation at atemperature of less than 60° C.
 14. The method of claim 13 whereinapplying actinic radiation comprises applying actinic radiation at atemperature of 25° C. or less.
 15. The method of claim 1 wherein themetal-containing catalyst comprises platinum.
 16. The method of claim 15wherein the metal-containing catalyst is selected from the groupconsisting of Pt(II) β-diketonate complexes,(η⁵-cyclopentadienyl)tri(σ-aliphatic)platinum complexes, andC₇₋₂₀-aromatic substituted (η⁵-cyclopentadienyl)tri(σ-aliphatic)platinumcomplexes.
 17. The method of claim 1 wherein the actinic radiation has awavelength of 600 nm or less.
 18. The method of claim 17 wherein theactinic radiation has a wavelength of from 200 to 600 nm.
 19. The methodof claim 18 wherein the actinic radiation has a wavelength of from 250to 500 nm.
 20. The method of claim 1 wherein the encapsulant is aliquid, gel, elastomer, or non-elastic solid.
 21. The method of claim 1wherein the encapsulant has a refractive index of at least 1.34.
 22. Themethod of claim 1 wherein the encapsulant has a refractive index of atleast 1.50.
 23. The method of claim 1 wherein the photopolymerizablecomposition comprises an organosiloxane.
 24. The method of claim 2wherein the photopolymerizable composition comprises an organosiloxane.25. The method of claim 1 wherein the photopolymerizable compositioncomprises a first organosiloxane having silicon-bonded hydrogen and asecond organosiloxane having aliphatic unsaturation.
 26. The method ofclaim 25 wherein the photopolymerizable material comprises anorganosiloxane comprising units of the formula:R¹ _(a)R² _(b)SiO_((4-a-b)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substituted,hydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; R² is a monovalent hydrocarbon group havingaliphatic unsaturation and from 2 to 10 carbon atoms; a is 0, 1, 2, or3; b is 0, 1, 2, or 3; and the sum a+b is 0, 1, 2, or 3; with theproviso that there is on average at least one R¹ present per molecule.27. The method of claim 26 wherein at least 90 mole percent of the R¹groups are methyl.
 28. The method of claim 26 wherein at least 20 molepercent of the R¹ groups are aryl, aralkyl, alkaryl, or combinationsthereof.
 29. The method of claim 28 wherein the R¹ groups are phenyl.30. The method of claim 26 wherein the R² groups are vinyl or 5-hexenyl.31. The method of claim 25 wherein the photopolymerizable materialcomprises an organosiloxane comprising units of the formula:R¹ _(a)H_(c)SiO_((4-a-c)/2) wherein: R¹ is a monovalent,straight-chained, branched or cyclic, unsubstituted or substituted,hydrocarbon group that is free of aliphatic unsaturation and has from 1to 18 carbon atoms; a is 0, 1, 2, or 3; c is 0, 1, or 2; and the sum ofa+c is 0, 1, 2, or 3; with the proviso that there is on average at leastone silicon-bonded hydrogen present per molecule.
 32. The method ofclaim 31 wherein at least 90 mole percent of the R¹ groups are methyl.33. The method of claim 31 wherein at least 20 mole percent of the R¹groups are aryl, aralkyl, alkaryl, or combinations thereof.
 34. Themethod of claim 33 wherein the R¹ groups are phenyl.
 35. The method ofclaim 1 wherein the photopolymerizable material comprises anorganosiloxane comprising the formulae:R¹ _(a)R² _(b)SiO_((4-a-b)/2) and R¹ _(a)H_(c)SiO_((4-a-c)/2) wherein:R¹ is a monovalent, straight-chained, branched or cyclic, unsubstitutedor substituted hydrocarbon group that is free of aliphatic unsaturationand has from 1 to 18 carbon atoms; R² is a monovalent hydrocarbon grouphaving aliphatic unsaturation and from 2 to 10 carbon atoms; a is 0, 1,2, or 3; b is 0, 1, 2, or 3; c is 0, 1, or 2; the sum a+b is 0, 1, 2, or3; and the sum of a+c is 0, 1, 2, or 3; with the proviso that there ison average at least one silicon-bonded hydrogen and at least one R²group present per molecule.
 36. The method of claim 35 wherein at least90 mole percent of the R¹ groups are methyl.
 37. The method of claim 35wherein at least 20 mole percent of the R¹ groups are aryl, aralkyl,alkaryl, or combinations thereof.
 38. The method of claim 37 wherein theR¹ groups are phenyl.
 39. The method of claim 35 wherein the R² groupsare vinyl or 5-hexenyl.
 40. The method of claim 1 wherein thesilicon-bonded hydrogen and the aliphatic unsaturation are present in amolar ratio of from 0.5 to 10.0.
 41. The method of claim 40 wherein thesilicon-bonded hydrogen and the aliphatic unsaturation are present in amolar ratio of from 0.8 to 4.0.
 42. The method of claim 41 wherein thesilicon-bonded hydrogen and the aliphatic unsaturation are present in amolar ratio of from 1.0 to 3.0.
 43. The method of claim 1 wherein thephotopolymerizable material comprises one or more additives selectedfrom the group consisting of nonabsorbing metal oxide particles,semiconductor particles, phosphors, sensitizers, antioxidants, pigments,photoinitiators, catalyst inhibitors, and combinations thereof.
 44. Alight emitting device prepared using the method of claim
 1. 45. Themethod of claim 12, wherein forming the encapsulant further comprisesthe step of heating at less than 150° C.
 46. The method of claim 45,wherein heating is at less than 100° C.
 47. The method of claim 46,wherein heating is at less than 60° C.
 48. A light emitting devicecomprising a light emitting diode and a photopolymerizable compositioncomprising a silicon-containing resin and a metal-containing catalyst,wherein the silicon-containing resin comprises silicon-bonded hydrogenand aliphatic unsaturation.
 49. The light emitting device of claim 48,wherein the metal-containing catalyst comprises platinum.
 50. The lightemitting device of claim 48, wherein the photopolymerizable compositionis at a temperature of from about 30° C. to about 120° C.
 51. The lightemitting device of claim 50, wherein the metal-containing catalystcomprises platinum.